(I'm reading The Insidious Dr Fu-Manchu and this is Chapter Twenty Four.)
He considers a few things including the overthrow of the Qing (Manchu) Dynasty by 'Young China', who Fu-Manchu had disparaged to his face and 'assuming that the name were not an assumed one, he clearly can have been no anti-Manchu, no Republican.' Well maybe, but just because you're called Scott doesn't make you pro-Scottish Independence. After a brief discussion of Young China* he suggests that in times of turmoil there will often by a third party, and that Fu-Manchu is a leader of such a group**.
He goes on to discuss Fu-Manchu's bases of operations and eventually stops expositing and tells us about their raid on the East End riverside building. Karamaneh insists that Petrie and Smith enter first and get her brother Aziz to safety. They enter, along with Inspector Weymouth of Scotland Yard.
"From the time when Nayland Smith had come from Burma in pursuit of this advance-guard of a cogent Yellow Peril, the face of Dr. Fu-Manchu rarely had been absent from my dreams day or night. The millions might sleep in peace—the millions in whose cause we labored!—but we who knew the reality of the danger knew that a veritable octopus had fastened upon England—a yellow octopus whose head was that of Dr. Fu-Manchu, whose tentacles were dacoity, thuggee, modes of death, secret and swift, which in the darkness plucked men from life and left no clew behind." Petrie really building up the atmosphere there.
They revive Aziz, but Fu-Manchu's laboratory has been stripped of it's contents. Next door they discover him, yet "the cunning mind was torpid—lost in a brutish world of dreams." He's been smoking opium. Karamaneh begs them not to enter. Weymouth pulls out his handcuffs and goes in.
"As though cast up by a volcano, the silken cushions, the inlaid table with its blue-shaded lamp, the garish walls, the sprawling figure with the ghastly light playing upon its features—quivered, and shot upward!" It's actually a trapdoor and Petrie passes out leaving us with a cliffhanger.
* "The Chinese Republican is of the mandarin class, but of a new generation which veneers its Confucianism with Western polish."
** Petrie enormously under-describes the complexity of the Xinhai Revolution.